JP2003066266A - Optical fiber connecting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber connecting method that reduces optical loss and that causes no outer diameter change nor bending deformation. SOLUTION: Two optical fibers 1, 2 are fusion spliced at the end faces, and then the fusion spliced part 3 so formed is heated, so that the two optical fibers coincide with each other in the mode field diameter in the fusion spliced part 3. In this method of connecting optical fibers, after the formation of the fusion spliced part 3, the two optical fibers 1, 2 are fixed with a fixing tool 5A and a tension meter 6, in a state where a tensile force in the axial direction is applied or unapplied to the fusion spliced part 3 using the tension meter 6, and subsequently the heat treatment is performed by a burner 4 for example.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber connecting method, and more particularly, to a mode field diameter (mode f).
The present invention relates to an optical fiber connection method for connecting two optical fibers having different yield diameters (MFDs) or two optical fibers having a small MFD when connecting the two end faces with low loss.

[0002]

2. Description of the Related Art In recent years, in optical communication systems, Wavelength Division Multiplexing (WD)
M) The transmission capacity is rapidly increasing due to the development of the transmission method. There is a strong demand for performance such as reduction of non-linear effects, reduction of chromatic dispersion, and reduction of chromatic dispersion slope for optical fiber lines wired in such a system having a large transmission capacity.

In order to meet this demand, the following distributed management line is under study. The dispersion management line is, for example, a single mode optical fiber (Simp
le Mode Fiber: end face of SMF (for example, 1300 nm zero-dispersion optical fiber) and dispersion-compensating optical fiber (for example, Dispersion) for compensating for dispersion and dispersion slope of this SMF.
Compensating Fiber: DCF, Dispersion Slope Com
pensating Fiber: SDCF, Reverse Dispersion Fiber
r: RDF, etc.) and the end face is fusion-bonded.
For example, it is about to be used for high-speed communication using light in the 1550 nm band.

By the way, in the case of the 1300 nm zero-dispersion optical fiber which is the single mode optical fiber exemplified above, the core is made of, for example, GeO ₂ -doped silica, the cladding is made of pure silica, and the wavelength is 1550 nm. The MFD is 9-11 μm. In the MFD magnifying single mode optical fiber, the MFD is 11 μm.
It is more than m.

On the other hand, in the case of a dispersion compensating optical fiber having a negative high dispersion characteristic, it is necessary to increase the relative refractive index difference to about 3%. Therefore, the core has a high concentration such as Ge
The silica doped with O ₂ and the clad are formed with silica doped with fluorine. And the core diameter is 2
It is about 3 μm, which is extremely smaller than the core diameter of the single-mode optical fiber. And the wavelength 155
The MFD at 0 nm has a value of about 5 μm. That is, both the core diameter and the MFD of the dispersion compensating optical fiber are smaller than those of the single mode optical fiber.

Therefore, even if the optical axes of the two optical fibers are made to coincide with each other by simply fusion-splicing the end faces of the above-mentioned two optical fibers, the MFD at the connecting portion will be the same.
The light loss occurs due to the light leakage based on the difference between the two. For example, a single mode optical fiber having an MFD of 10 μm,
If the fusion-compensated optical fiber having an MFD of 5 μm is simply fusion-spliced with its optical axis aligned, the optical loss at the fusion-spliced portion will be about 1.94 dB.

To prevent the occurrence of optical loss in such a fusion spliced portion, the TEC method (Thermally Defused Ex
The optical loss is reduced by applying the panded core method). In this TEC method, heat treatment is applied to the fusion splicing portion,
The dopant in the core is diffused into the cladding to substantially expand the core and MFD. For example, when the TEC method is applied to the fusion spliced portion of the single mode optical fiber and the dispersion compensating optical fiber, the softening temperature of the cladding (fluorine-doped) of the dispersion compensating optical fiber is ), The rate at which the dopant (GeO ₂ ) in the cores of both optical fibers diffuses into their respective claddings is faster in dispersion compensating optical fibers than in single mode optical fibers. Therefore, in the process of heat treatment, the dopant of the core of the dispersion compensating optical fiber diffuses preferentially, and the substantial expansion of the core diameter progresses at the fusion splicing portion, and the core diameter of the dispersion compensating optical fiber becomes uniform. It matches the core diameter of the mode optical fiber. That is,
Since the MFDs match, a reduction in optical loss between both optical fibers is realized.

In this way, the optical loss at the fusion splicing part is reduced. Regarding the connection of the optical fibers, as described above, not only the connection of different kinds of optical fibers having different core diameters and MFDs, but also the optical fibers of the same kind are mutually connected for the purpose of adjusting the length and characteristics of the entire optical line. You will also need to connect. For example, the same kind of dispersion compensating optical fiber having an extremely small MFD and therefore an extremely small core diameter may be connected. Also in this case, the mutual end faces of the two optical fibers are fusion-spliced by using, for example, a fusion splicer.

However, in this case, since the core diameter is very small, there is a problem that the optical loss in the fusion splicing portion becomes large even if the cores are slightly displaced from each other. Further, when, for example, a discharge fusion splicer is used at the time of fusion splicing, even if the discharge condition of the discharge fusion splicer is optimized, at the time of the recent connection of the thin wire core, a sufficient reduction of optical loss can be achieved. Not realized.

Therefore, even in the case of such a connection, the TEC method is applied to the formed fusion spliced portion to diffuse the dopant in the core to expand the MFD in the fusion spliced portion, and thus the above-mentioned axis deviation is caused. It has solved the occurrence of light loss due to.

[0011]

By the way, for example, in the above-mentioned dispersion management line, when it is used for an optical submarine cable, the fusion splicing part may have low loss and high strength at the same time. Needed. Conventionally, the following measures have been taken to increase the strength of the fusion-spliced portion. That is, before performing the fusion splicing, V of the fusion splicer in which the fiber cutter and the optical fiber are arranged.
The surface of the optical fiber is coated with a resin protective layer in order to reduce or remove the influence of factors that deteriorate the strength of the optical fiber, such as contact with a groove or a fiber clamp that fixes the optical fiber.

However, when the above-mentioned resin protective layer is formed, tackiness remains to a considerable extent on its surface. Therefore, during fusion splicing, the straightness of the optical fiber may be impaired and meander, or the fibers may not advance at the timing when they should advance to each other, and as a result, the amount of deviation of the core is larger than that when the resin protective layer is not formed. It becomes very large, and the optical loss in the fusion splicing part becomes large.

In particular, in order to fusion splice an optical fiber having a small MFD with high strength, in the case where the above-mentioned resin protective layer is formed on the optical fiber for fusion splicing, in order to reduce optical loss, The fusion splicing must be done so that even slight misalignment of the core does not occur, but in practice it is a very difficult task. Thus, it can be said that forming a resin protective layer on the surface of the optical fiber to be connected is an effective means for increasing the strength of the fusion spliced portion, but on the other hand, in the case where the resin protective layer is not formed. In contrast to this, the optical loss in the fusion spliced portion is further increased.

As the heat treatment to be performed after fusion splicing, discharge, hydrogen / oxygen burner flame or propane / oxygen burner flame is usually employed. However, in the case of electric discharge heating, only very local heating is realized for the fusion spliced portion, and the heating temperature is high, so that it is treated in a relatively short time and then rapidly cooled. . Therefore, the diffusion state of the dopant in the core is likely to be unstable, and there is a problem that strain is accumulated in the glass of the fusion-bonded portion.

Moreover, in the discharge heating, it is quite difficult to realize the optimization of the discharge conditions, and it is difficult to realize the control to an appropriate heating temperature and the optimization of the heating location. Therefore, when the discharge heating is repeated a plurality of times, there arises a problem that the outer diameter fluctuation (so-called constriction) occurs in the fusion spliced portion to make the diameter smaller, and at the same time, the strength also decreases. On the other hand, in the case of heating with a burner flame, as compared with discharge heating, it is easier to perform appropriate temperature control and it is easier to appropriately narrow down the heating location. However, on the other hand, since the optical fiber is arranged sideways and the burner flame is blown to the softened fusion splicing part, the fusion splicing part is bent by the pressure of the flame and the self-weight of the optical fiber. The light loss may increase due to deformation.

To solve such a problem, a method has been proposed in which a heat treatment is applied to the fusion splicing portion while applying axial tension to the fusion splicing portion. However, depending on the magnitude of the applied tension, the softened fusion splice stretches, and as with repeated discharge heating, a constriction occurs in the fusion splice, and the strength of the fusion splice decreases. Is caused, and the variation in strength becomes large.

In the optical fiber in such a state, when the fusion spliced portion is bent, stress is concentrated there, which may cause a breakage accident. The present invention reduces the optical loss in the fusion splicing part at the time of connecting the optical fibers,
It is possible to realize high strength of the fusion spliced part and minimization of strength variation, and it is possible to realize a suitable optical fiber to be applied to connection of different kinds of optical fibers having different MFDs, and connection of same kind of optical fibers having extremely small MFD. The purpose is to provide a connection method.

[0018]

In order to achieve the above object, in the present invention, two optical fibers are fusion-spliced at their end faces to form a fusion splicing portion, and then the fusion splicing is performed. In a method of connecting optical fibers in which a fusion splicing portion is subjected to heat treatment to match the mode field diameters of the two optical fibers located in the fusion splicing portion, the heat treatment is performed after the fusion splicing portion is formed. Prior to carrying out the above, the two optical fibers are fixed by a fixing jig with or without applying axial tension to the fusion splicing part, and then the heat treatment is applied to the fusion splicing part. A method of connecting optical fibers is provided.

In this case, the applied tension is preferably 1176 mN or less, more preferably 0.98 to 49 mN.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, first, the end faces of two optical fibers to be spliced are fusion-spliced to form a fusion spliced portion. Regarding fusion splicing, for example, a discharge fusion splicer may be used as in the conventional case. At this point, at the fusion splice, in most cases M of two optical fibers is
FD does not match. Next, the fusion splicing portion is subjected to a heat treatment described later to perform an MFD matching operation of the two optical fibers. The procedure will be described below with reference to FIG.

Step 1: First, the fusion splicing portion 3 of the optical fibers 1 and 2 is placed at the position of the heating means 4, and one optical fiber (optical fiber 1 in FIG. 1) is fixed, for example, by a clamp. Hold and fix with jig 5A. At this time, the gripping force with respect to the optical fiber 1 by the fixing jig 5A is set to a value larger than the applied tension described later.

Step 2: Then, the optical fiber 2 is held by the fixing jig 5B arranged on the optical fiber 2 side. The gripping state at this time is not a strong gripping state in which the optical fiber 1 is fixed, but a loose gripping state in which the optical fiber 2 can move the fixing jig 5B.

Step 3: Then, the optical fiber 2 is set on the tensiometer 6 arranged outside the fixing jig 5B, a predetermined tension is applied to the optical fiber 2, and the optical fiber 2 is left in that state. To fix. Therefore, the fusion splicing part 3 is
The fixing jig 5A and the tensiometer 6 are fixed while being continuously pulled by the tension applied from the tensiometer 6. At this time,
Since the gripping force of the fixing jig 5A is larger than the applied tension, the optical fiber 1 will not come off from the fixing jig 5A. In addition,
In this procedure 3, without applying tension to the optical fiber 2, the optical fiber 2 is simply moved by the tensiometer 6 and the fixing jig 5B.
May be fixed only.

Step 4: And finally, the heating means 4 is activated to heat the fusion splicing part 3 and the MFD of the two optical fibers.
Match. As the heating means 4 used at this time,
It is preferably a burner flame. This is because, as described above, the control of the heating temperature is capacity, and it is possible to selectively heat only the target portion relatively accurately.

By this heat treatment, in the fusion splicing part 3, of the MFDs of the optical fiber 1 and the optical fiber 2, the smaller MFD expands in a trumpet shape to match the larger MFD. As a result, reduction of optical loss is realized. In that case, if the heating temperature is constant, as the heating time elapses, the diffusion of the dopant proceeds and the diameter expansion of the small MFD progresses, and the optical loss at the fusion splicing portion also decreases correspondingly. To go. The optical loss becomes minimum when the sizes of the two MFDs completely match.

In order to monitor this state, in the present invention, it is preferable to perform the connection work under the following system. As shown in FIG. 2, for example, the dummy fiber 7 fused and connected to the other end of the optical fiber 2 in FIG.
OTDR8 is connected via. Further, the heating means (for example, a burner) 4 is connected to a heating means control unit 9 which performs on / off of a heating state, temperature control, time control and the like.

Then, the OTDR 8 and the heating means controller 9
Are respectively connected to the feedback control unit 10 including the signal comparison unit 10A and the memory unit 10B in which the target setting value of the optical loss is stored. In the system shown in FIG. 2, the inspection light from the OTDR 8 enters the dummy fiber 7 and the optical fiber 2, enters the optical fiber 1 from the optical fiber 2 through the fusion splicing part 3, and then the dummy fiber 7 and the optical fiber 2. The return light from the fiber 2, the fusion splicing part 3, and the optical fiber 1 returns to the OTDR 8 in this order.

The OTDR 8 detects the returned lights and converts them into electric signals. Then, the light intensity of the fusion splicing section 3, that is, the degree of light loss is monitored every moment, and the monitor signal is input to the feedback control section 10.
In the feedback control unit 10, the signal comparison unit 10A compares the target setting value of the optical loss stored in the memory unit 10B with the monitor signal from the OTDR 8.

Then, when the optical loss in the fusion splicing portion 3 displayed by the monitor signal is larger than the target set value,
When a heating continuation operation signal is transmitted to the heating means control unit 9 and when it is equal to or lower than the target set value, a heating end signal is transmitted to the heating means control unit 9 to heat. The heating operation by means 4 is terminated. According to this system, it is possible to prevent excess and deficiency of the heat treatment time, reliably match the two MFDs, and control the treatment time so that the optical loss in the fusion splicing portion 3 becomes a target set value. it can.

In the above connection method, the optical fiber expands and expands during the heating process, and contracts and contracts during the cooling process. However, in the present invention, since the optical fibers 1 and 2 are fixed in a tension-applied state between the fixing jig 5A and the tensiometer 6 (at a constant interval) during the whole heating process, the optical fibers are always fixed. It maintains a straight state. That is, since the fusion splicing portion 3 does not stretch during the heating process, the diameter reduction (constriction) due to the outer diameter variation of the fusion splicing portion does not occur,
Further, the strength of the fusion spliced portion does not decrease due to the reduction in diameter.

Here, in step 3, the tension applied to the fusion splicing part 3 is preferably a value of 1176 mN or less. This is because if the applied tension is larger than 1176 mN, the optical loss of the fusion spliced part 3 after the heat treatment will be drastically increased. In the procedure 3, even if the optical fibers 1 and 2 are simply fixed by the fixing jig 5A and the fixing jig 5B without applying tension to the fusion splicing portion 3 and then subjected to heat treatment, the optical loss is reduced. Can be realized. This is because glass generally undergoes elastic deformation of about 0.1% during heat treatment, so the optical fibers 1 and 2 slightly shrink toward the respective fixing jigs during heat treatment, resulting in fusion splicing. It is considered that this is because the state of tension application automatically appears in the section 3.

Further, the outer diameter variation (thinning) of the fusion splicing portion 3
Taking into consideration the problem of strength reduction and the like, it is more preferable to set the applied tension in the procedure 3 within the range of 0.98 to 49 mN. This is because when the applied tension is set within the above range, it is possible to reliably suppress the occurrence of the outer diameter variation and the strength reduction in a state where the optical loss of the fusion splicing portion is reduced to, for example, 0.1 dB or less.

[0033]

EXAMPLES Example 1 The following connection work was performed in the mode shown in FIGS. Optical fiber (outer diameter 125 μm, MFD 11.4 μm, core dopant is GeO ₂ (single mode optical fiber) 1
And an outer diameter of 125 μm, an MFD of 5.7 μm, and a core dopant GeO ₂ (dispersion compensating optical fiber) 2
Prepared.

These optical fibers were set in a discharge fusion splicer, and their end faces were fusion spliced under the conditions of arc discharge voltage of 1 kV, discharge current of 17.9 mA, discharge time of 2.3 seconds, and pushing amount of 1 μm. The fusion splicing part 3 was formed. The optical loss of this fusion spliced part was about 1.6 dB. Then, the optical fiber 1 was gripped and fixed by the clamp 5A. The gripping force at this time was set to about 1960 mN.

Then, the optical fiber 2 was lightly gripped by the clamp 5B with a gripping force of 9.8 mN or less, and further fixed with tension applied by the tensiometer 6. At this time, the applied tension is 1
It was changed at intervals of 96 mN (20 gf). Then burner 4
Was activated to heat the fusion spliced portion 3. Then, when the light loss value reached the target set value, the burner 4 was operated again to stop the heating.

The optical loss in the fusion spliced portion 3 after heating was measured, and the result is shown in FIG. 3 as a relationship with the applied tension. As is apparent from FIG. 3, when the applied tension in the procedure 3 is larger than 1176 mN, the optical loss increases rapidly. Further, even when the optical fiber is simply fixed without applying the tension (when the applied tension is 0 in FIG. 3), the optical loss is small and is about 0.1 dB.

Example 2 Outer diameter 125 μm, MFD 12 μm, core dopant G
eO ₂ MFD magnifying single mode optical fiber 1
And outer diameter 125 μm, MFD 4.9 μm, core GeO ₂
A dispersion compensating optical fiber 2 was prepared, which was made of highly-doped silica and the cladding was made of fluorine-doped silica.

The end faces of the respective optical fibers were discharge fusion-spliced under the same conditions as in Example 1 to form fusion-spliced portions. Then, in the same procedure as in Example 1, after applying the various applied tensions shown in Table 1 to the fusion spliced portion, the optical fibers 1 and 2 were fixed, and the fusion spliced portion was burner heated in each case. .

Then, in the system shown in FIG.
The optical loss of the fusion spliced part was monitored with a laser beam having a wavelength of 1500 nm, and the heat treatment was stopped when the optical loss was minimized. Then, the outer diameter of the fusion spliced portion and the amount of bending deformation were measured using a laser outer diameter measuring device. The following results are collectively shown in Table 1.

[0040]

[Table 1]

The following are clear from Table 1. (1) In the case of Examples 3, 4, and 5 in which the tension applied before the heat treatment was within the range of 0.98 to 49 mN, the light loss was first less than 0.1 dB, and the outer diameter was The amount of fluctuation and the amount of bending deformation are very small.

In the case of Example 2 in which the applied tension is 98 mN, the optical loss is larger, and the variation of the outer diameter and the amount of bending deformation are also greater than those of Examples 3-5. From this, if the applied tension before the heat treatment is set to 0.98 to 49 mN, the optical loss at the fusion splicing part can be greatly reduced, and at the same time the outer diameter fluctuation (constriction) and the bending deformation are almost certainly generated. It can be seen that it can be suppressed and is very suitable.

(2) In Table 1, in the case of Example 6 in which the optical fiber is simply fixed and no tension is applied, the good results are obtained as in the case of Examples 3 to 5. Therefore, when a reproducibility test was performed on both Examples, Examples 3 to 5 were performed.
In the case of, the same result as in Table 1 was obtained with a probability of almost 100%. However, in the case of Example 6, the probability of reproducing the results of Table 1 was about 70%.

[0044]

As is apparent from the above description, according to the method of the present invention, it is possible to connect heterogeneous optical fibers having different MFDs or optical fibers of the same kind having a very small MFD with low loss. This is an effect obtained by applying tension to the fusion splicing portion and then performing the heat treatment before the heat treatment when the fusion splicing portion is heated to match the MFD of the optical fiber. .

At this time, the applied tension is preferably 1176 mN or less, and when it is 0.98 to 49 mN, the optical loss is greatly reduced, and the outer diameter fluctuation of the fusion splicing portion ( Occurrence of constriction) and bending deformation can be suppressed almost certainly, and it is also effective in preventing a decrease in strength of the fusion-spliced portion.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a connection method of the present invention.

FIG. 2 is a schematic diagram showing an example of a system for monitoring a connection state in the connection method of the present invention.

FIG. 3 is a graph showing the results of Example 1.

[Explanation of symbols]

1, 2 Optical fiber to be connected 3 Fusion splicing part 4 Heating means (burner) 5A, 5B Optical fiber fixing jig (clamp) 6 Tensiometer 7 Dummy fiber 8 OTDR 9 Heating means controller 10 Feedback control unit 10A signal comparison section 10B memory section

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroaki Onuma             2-6-1, Marunouchi, Chiyoda-ku, Tokyo             Kawa Electric Industry Co., Ltd. (72) Wataru Komatsu             2-6-1, Marunouchi, Chiyoda-ku, Tokyo             Kawa Electric Industry Co., Ltd. F-term (reference) 2H036 MA15 MA18

Claims (5)

[Claims] 1. A fusion splicing part is formed by splicing two optical fibers at their end faces to form a fusion splicing part, and then the fusion splicing part is subjected to heat treatment to be positioned at the fusion splicing part. In an optical fiber connecting method for matching the mode field diameters of two optical fibers, after forming the fusion splicing portion, an axial tension is applied to the fusion splicing portion before the heat treatment. A method for connecting optical fibers, characterized in that the two optical fibers are fixed by a fixing jig in a state of not being applied, and then the heat treatment is applied to the fusion spliced portion. 2. The method for connecting optical fibers according to claim 1, wherein the tension has a value of 1176 mN or less. 3. The method for connecting optical fibers according to claim 1, wherein the tension is 0.98 to 49 mN. 4. The optical fiber connecting method according to claim 1, wherein the heating means used in the heat treatment is a burner means. 5. The light is introduced from one end of one optical fiber while the fusion splicing portion is subjected to the heat treatment, and a change in the light intensity when the introduced light passes through the fusion splicing portion is performed. The method of connecting an optical fiber according to any one of claims 1 to 4, which is monitored.